The present invention relates to an electro-flocculation process and apparatus for water treatment and especially for the production of drinking water.
More particularly, the present invention relates to a flocculation process and apparatus using trivalent ions such as those of aluminum and iron, accompanied by in-situ disinfection, without the need for flotation and sedimentation steps as described in the prior art.
Surface water intended for drinking water supply at present are treated for particle removal until extremely low turbity is reached.
Tertiary or advanced wastewater treatment is a requirement in some countries prior to disposal of water, and is one of the most important demands for wastewater reuse. Common tertiary or advanced wastewater treatment schemes include chemical coagulation (flocculation) of secondary clarifier effluent followed by sedimentation, and/or filtration and disinfection. The physical processes associated with classical coagulation are rapid mixing (when chemical coagulants are quickly and uniformly dispersed in the water and particles are destabilized), and then slow mixing or flocculation (when particles slowly aggregate and form settleable or filterable flocs). Chemical flocculation processes are known to aggregate wastewater constituents having a size ranging from 0.1 xcexcm to about 10 xcexcm.
Aluminum or iron salts may be added at various stages in a potable water or wastewater treatment plant to enhance solids removal. The metal salt (coagulant or flocculant) destabilizes colloidal solids that would otherwise remain in suspension and thus can be used to improve effluent quality. In spite of the increased use of chemical coagulants, coagulation theory still fails in providing the use of the process in an optimal manner, particularly under transient conditions. This could lead to diminished effluent quality, increased chemical costs by routine overdosing, or both. An improved coagulation process would alleviate its adverse effects on sludge dewaterability and disposal.
Coagulation has been the subject of much research, some of which have proposed coagulation in the context of water treatment. However, wastewater treatment differs from potable water treatment in several ways: particulate matter is present in substantially greater concentrations in wastewater; the average particle size is also greater. These factors are likely to affect both coagulant demand and flocculation behavior. The particulates to be removed include a much greater proportion of organic material than in the case of water treatment for drinking purposes. The more hydrophilic surfaces of these particles may react differently to a coagulant.
The mechanisms of coagulation by Al+3 salts may involve some chemical factors including the hydrolyzing and polymerizing tendencies of the Al+3 ion, the adsorptivity of such aluminum hydroxide species; the solubility of such species; the nature and extent of aluminum hydroxide precipitation, including interaction with other colloidal surfaces and effects of other solutes of surface properties on the metal hydroxide species. In light of this apparent complexity, it is not surprising that a number of different models have been proposed to explain the way aluminum salts can destabilize colloids.
The earliest explanation on particle destabilization considered only double-layer compression by Al+3 ions as such. Other explanations suggested that the hydrolysis products of aluminum play a more important role in particle destabilization. More current explanations of coagulation with alum or iron flocculants hypothesize the existence of two distinct mechanisms, which are charge neutralization and adsorption of positively charged, polynuclear aluminum species to the particles; or the physical enmeshment of particulates in an aluminum hydroxide precipitate.
Electrochemical processes in water treatment which are gaining ground are electrofloatation and disinfection. The main advantage of the electrochemical process of disinfection is the production of disinfective chemicals in situ in the treatment device.
Floatation is a gravity separation process in which air bubbles are attached to individual solid particles, thereby reducing their density so that they float to the surface of the liquid. In the electrofloatation process gas bubbles are generated by electrolysis of the liquid. A 5-20V direct current is used at a current density of approx. 100 amp/m2 of electrode The particular attraction of floatation, as compared to sedimentation, is that particles"" rising rate is generally much higher than their settling rate, so that the size of the unit for a given duty is approximately one third of that of a clarifier.
In WO 95/15295, EP 668 244, GB 2,045,803 and DE 3,641,365, as well as in WO 97/35808, there are described electroflocculation processes involving the release of aluminum ions through an electrolytic cell. The first four documents refer to the electroflocculation-flotation process with regard to wastewater and teach the formation of trivalent ions in situ by electrolysis of a metallic aluminium or iron electrode, while the latter document, although referring to the use of said process for producing drinking water, does not teach the apparatus or method which would enable said process to be carried out
With this state of the art in mind, it is an object of the present invention to combine known electrochemical processes for water and wastewater treatment with the introduction of trivalent ions directly into the solute by electrolysis and the use of specially designed apparatus, as defined and described hereinafter.
More particularly, the present invention provides an electroflocculation process for water and wastewater treatment comprising providing a stack of electrically-conducting perforated spaced-apart plates serving as metallic electrodes, said electrodes being selected from the group consisting of metallic aluminum electrodes and metallic iron electrodes, and said plates being suspended in a vessel with their peripheral edges in proximity to a wall of said vessel, each plate being electrically insulated from adjacent plates and having apertures misaligned relative to apertures of adjacent plates; and supplying a voltage to said electrodes in the range of between 2 to 60 volts to form trivalent ions in situ by electrolysis of said metallic electrodes, which trivalent ions function as coagulation agents, wherein water to be treated is caused to ascend sequentially through apertures provided in said stack of parallel metallic plates serving as said electrodes, each plate being charged with a polarity opposite to plates adjacent thereto, apertures of adjacent plates being vertically misaligned, whereby turbulent flow and mixing is created by the sequential passage of said water to be treated from the apertures of the lowest of said plates to the apertures of the plates thereabove to exit from the highest of said plates.
In preferred embodiments of the present invention the distance between said plates is between 0.5 and 1.5 cm. The present invention also provides an apparatus for water and wastewater treatment by the electroflocculation process, comprising
a) a processing vessel having a lower inlet for impure water, and an upper outlet for purified water,
b) a stack of electrically-conducting perforated spaced-apart plates serving as metallic electrodes, said electrodes being selected from the group consisting of metallic aluminum electrodes and metallic iron electrodes, and said plates being suspended in said vessel with their peripheral edges in proximity to a wall of said vessel, each plate being electrically insulated from adjacent plates and having apertures misaligned relative to apertures of adjacent plates; and
c) a first and a second electrically conducting member, said first member being electrically connected to the upper plate of said stack and to alternate lower plates and insulated from other plates, said second member being electrically connected to plates electrically unconnected to said first member and insulated from plates connected to said first member, said conducting members being provided with terminals outside said vessel for connection thereto of an electric power supply, supplying voltage in the range of between 2 and 60 volts for forming trivalent ions in situ by electrolysis of said metallic electrodes which trivalent ions function as coagulating agents within the vessel.
In U.S. Pat. No. 3,841,483 (D1) there is described a water purification system including a flow-through water purification apparatus for separating colloidal particles suspended therein.
While said patent describes the use of a plurality of mutually spaced oppositely charged plates arranged in a manner superficially similar to that of the present invention, said patent teaches and describes a process based on an entirely different principle and mechanism in that said patent describes the application of a voltage of up to around 200 volts to induce positively charged ionization thus causing electrophoretic action leading to coagulation of the colloid particles.
Similarly, while said patent mentions the possible use of metals such as iron and aluminum said metals are mentioned in the context of the possible use of known metals such as iron, aluminum, copper and bismuth and even carbon, which is not a metal, or any combination of various alloys and thus it is clear that the mere mention of iron and aluminum in said patent is not intended to suggest or teach the intentional forming of trivalent ions in situ by electrolysis of said metallic electrodes which trivalent ions function as coagulating agents within the process.
In LU 88 244 (D2) there is described a process and apparatus for water purification which publication describes the use of a plurality of mutually spaced oppositely charged plates. However, as can be seen from the figures of said patent the plates are arranged vertically rather than horizontally and results in stagnant volumes and sedimentation or floatation because of the positioning of the vertical plates. Thus said patent does not teach or suggest the process or apparatus of the present invention wherein turbulent flow and mixing is created by the sequential passage of water to be treated from the apertures of the lowest of said plates to the apertures of the plates thereabove to exit from the highest of said plates
The classical coagulation agent Al+3 ions is usually obtained by dissociation of dissolved alum salt [Al2(SO4)3] containing 13 to 18 molecules of bound water. The molecular weight of the alum is 666.7 and the aluminum (m.m. 54) from this represents only 8% as the active component of the flocculation process. The removal efficiency is a function of coagulant dosage, and the pH level, as well as the mixing (dispersion) condition.
According to the present invention and in contradistinction to the classical approach to coagulation and flocculation, trivalent ions are introduced directly in situ.
Coagulant chemicals are purchased either in dry or liquid form. Dry chemicals require fairly elaborate dissolving and feeding equipment. For effective chemical usage, mixing with the process water stream must be rapid and complete. A high speed mixer or equivalent static mixer is required.
For each milligram/liter of alum added, the sulfate content of the water increases by 0.5 mg/l. This may be detrimental if the raw water sulfate content is high, or if the plant uses ion exchange for nitrate removal.
The present invention provides for the direct introduction in the raw water stream of only the Al+++ of Fe+++ ions in an electrochemical process. The mixing conditions according to the present invention are easier than in the classical methods. In addition, because the raw water OHxe2x88x92 contents will increase, an effective oxidizing-disinfecting stage is created in the same unique treatment-step.
Besides the savings on flocculant cost, the concentration of aluminum in the effluent is minimized, thus reducing potential hazardous effects resulting from high aluminum concentrations in water.
The process of the present invention is simple in the aspects of setting up, operation and maintenance. Consequently it is applicable also for small communities which are unable to employ skilled specialists for running a costly and complex process.
While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.
A series of laboratory-scale experiments were performed. The following results were obtained (as are shown in Table 1 hereinafter).
Sample of raw wastewater having initial turbidity of 400 NTU and TSS of 660 mg/l was conventionally treated with a dosage of 40 mg/l alum. Reductions to 65 NTU and TSS to 135 mg/l were obtained.
By electroflocculation of a second sample of the same raw wastewater according to the present invention, using a metallic electrode for producing trivalent ions, 2.5 NTU and TSS of 8.8 mg/l were obtained.
Another series of electroflocculation experiments was carried out according to the present invention and the results are presented in Table 1 hereinafter.
As can be seen, with increase in voltage and current, turbidity and total particle count, as well as total suspended solids is reduced when using the apparatus of the present invention.
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with.the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.